1. Field of the Invention
Mechanical presses that use dies to manufacture parts, produce heat during their operation. As a result, the temperatures of respective parts increase, and if the mechanical press is stopped, its temperature drops to the ambient temperature by radiation of heat. However, the degrees of temperature change differ among the respective components constituting the mechanical press. Thus, differences of the temperatures are produced among the respective parts, so that differences in the amount of heat expansion occur among the respective parts. The position of the bottom dead center of the slide (that is, the lowest position of the slide during a stroke by the press) is changed because of the differences in the amount of heat expansion. Consequently, the die height of the mechanical press is changed. The change of the die height adversely affects the accuracy of parts which are produced by the press. The present invention relates to a method for minimizing the change of the die height, or the change of the point of bottom dead center of the slide, so the press can produce parts with a high degree of accuracy.
2. Description of Related Art
The following three methods for minimizing the change of the bottom dead center point of the slide have been proposed:
a) Japanese Patent Publication No. 30569/1989 discloses a method in which oil of a predetermined temperature is splashed on connecting rods that experience a large change of the temperature. However, when this method is applied, it is difficult to maintain the position of the bottom dead center of the slide accurately when the mechanical press is operated in such a manner that changes in the amount of generated heat occur. Changes in heat generation may occur, for example, when the SPM (press speed in strokes per second) is changed or when the cycle of startup/stop operation, and the duration of the cycles are changed.
b) Japanese Patent Publication No. 55056/1989 discloses a method in which a stopper block is disposed between a slide and a bed so that they correspond to each other. The stopper block is hit each time the bottom dead center is reached, in order to restrict the position of the bottom dead center point. According to this method, the position of the bottom dead center can be restricted accurately if the elasticity of the stopper block is high. However, an increase in elasticity causes variations in the load applied to the mechanical press to increase. Therefore, the mechanical press may be damaged. If the elasticity of the stopper block is low, it is not possible to control the position of the bottom dead center accurately.
c) FIG. 7 illustrates another method for maintaining the position of the bottom dead center at a constant position. The method uses a die-height adjusting device employing a threaded connecting rod 11P, a threaded lock nut 22P, a slide 4, an adjusting screw 16P, a worm shaft 18, a worm wheel 19P and other parts. Adjusting screw 16P can be screwed into or out of connecting rod 11P when lock nut 22P is spaced apart from the bottom end of connecting rod 11P, but when lock nut 22P is screwed tightly against the bottom end of connecting rod 11P as shown it forces connecting rod 11P downward by a minute amount which is nevertheless sufficient to eliminate the normal clearance between the threads of connecting rod 11P and adjusting screw 16P, jamming the threads against one another. Although the lock nut 22P can be tightened to eliminate the normal clearance between the threads on the connecting rod 11P and the adjusting screw 16P, the die height cannot be adjusted until the lock nut 22P is loosened. Because it is not possible to restore the normal clearance of the threads unless the lock nut 22P is loosened, the mechanical press must be stopped during the adjustment of the die height. Further, the amount of adjustment for accurately maintaining the bottom dead center point at a constant position under changing temperature conditions during operation, is fine as compared with the amount of the adjustment of the die height which is ordinarily performed during an exchange of dies, or the like. Also, control of the lock nut 22P is necessary. An accurate adjustment therefore is difficult and takes a long time to perform.
As described above, there have been no methods in which the changes of the bottom dead center point can be adjusted accurately, regardless of whether the press is running or stopped, and previous methods lack part of the required functions.